Atom-Thick Interlayer Made of CVD-Grown Graphene Film on

Nov 27, 2017 - After 1500 discharge–charge cycles, the Li–S cell assembled with 2G-PP presented a capacity decay rate of 0.026% per cycle (Figure ...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Atom-Thick Interlayer Made of CVD-Grown Graphene Film on Separator for Advanced Lithium−Sulfur Batteries Zhenzhen Du,† Chengkun Guo,‡ Linjun Wang,§ Ajuan Hu,† Song Jin,† Taiming Zhang,† Hongchang Jin,† Zhikai Qi,† Sen Xin,⊥ Xianghua Kong,*,‡ Yu-Guo Guo,∥ Hengxing Ji,*,† and Li-Jun Wan*,†,∥ †

Department of Materials Science and Engineering, CAS Key Laboratory of Materials for Energy Conversion, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Technology of China, Hefei 230026, China ‡ Department of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China § Hefei National Laboratory for Physical Sciences at the Microscale and Center for Micro- and Nanoscale Research and Fabrication, University of Science and Technology of China, Hefei 230026, China ∥ Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences and Beijing National Laboratory for Molecular Sciences, Beijing 100190, China ⊥ Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Lithium−sulfur batteries are widely seen as a promising next-generation energy-storage system owing to their ultrahigh energy density. Although extensive research efforts have tackled poor cycling performance and self-discharge, battery stability has been improved at the expense of energy density. We have developed an interlayer consisting of two-layer chemical vapor deposition (CVD)-grown graphene supported by a conventional polypropylene (PP) separator. Unlike interlayers made of discrete nano-/microstructures that increase the thickness and weight of the separator, the CVD-graphene is an intact film with an area of 5 × 60 cm2 and has a thickness of ∼0.6 nm and areal density of ∼0.15 μg cm−2, which are negligible to those of the PP separator. The CVD-graphene on PP separator is the thinnest and lightest interlayer to date and is able to suppress the shuttling of polysulfides and enhance the utilization of sulfur, leading to concurrently improved specific capacity, rate capability, and cycle stability and suppressed self-discharge when assembled with cathodes consisting of different sulfur/carbon composites and electrolytes either with or without LiNO3 additive. KEYWORDS: CVD-graphene film, separator, interlayer, lithium−sulfur battery, polysulfide

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host,5−11 investigating new electrolytes or electrolyte additives,12−14 and optimizing the discharge−charge depth15 in order to suppress the shuttling of polysulfides. However, these soluble species do not remain within the cathode region because they can diffuse through the large pores (diameter of ∼100 nm) of the porous separator and shuttle between the S cathode and the Li anode, leading to a high capacity decay rate and a low Coulombic efficiency.16,17 Building a conductive interlayer onto the separator is a straightforward method to mitigate the migrating of polysulfides so that the trapped active material could be reused. For example, Su and Manthiram have designed an interlayer made of free-standing carbon paper inserted between the separator and the S cathode.18 Alternatively, Liu and co-workers have developed a hybrid anode that contains an interlayer film made

INTRODUCTION The increasing popularity of portable electronic devices and electric vehicles has led to the high demand for high energy and power density storage systems. Unfortunately, traditional cathode materials based on lithium transition-metal oxides and phosphate are unable to satisfy these requirements.1,2 Lithium−sulfur (Li−S) batteries are a promising alternative as they have a high theoretical energy density of 2567 W h kg−1, and the natural abundance, low cost, and environmentally benign nature of elemental S. However, their practical application is hindered due to low S utilization, fast capacity decay rate, poor Coulombic efficiency, and short cycle life. These issues derive from the low conductivity of elemental S and its final discharge product Li2S, the volume expansion of sulfur during discharge, and the dissolution and shuttle effect of highly soluble polysulfides (Li2Sx, x = 4, 6, and 8) generated during cycling.3,4 Recently, there has been much work devoted to resolving these issues, such as tailoring the structure and surface chemistry of porous carbon or conductive polymer © XXXX American Chemical Society

Received: September 19, 2017 Accepted: November 27, 2017 Published: November 27, 2017 A

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces of graphite powder placed between the Li anode and the separator.19 More recently, interlayer-integrated separators have been developed using a slurry-coating method or a vacuumfiltration process.20−33 Generally, Li−S batteries with interlayer coatings on the separator have improved specific capacities and cycling stabilities to batteries with no coating. There is a benefit to having interlayer coatings; however, the large amount of binder needed in the slurry-coating method reduces the electrical conductivity of the interlayer, and the unsatisfactory thickness and high loading of the interlayer undermines the energy density. Moreover, the large-scale deployment of functional separators with improved electrochemical performance demands an advanced fabrication technique,34,35 and fundamental research to develop interlayers on separators that approach the thickness lower limit is of great interest and may lead to technological progress in industry. Graphene film formed by chemical vapor deposition (CVDG) gives a > 98% single-layer coverage with a lateral size of tens of centimeters and has high electrical conductivity, flexibility, mechanical strength, and excellent chemical resistance.36 The film thickness can be changed by stacking multiple single-layer CVD-G sheets by transfer.37,38 The introduction of a monolayer or multilayer CVD-G film onto the separator would engender the properties of CVD-G to the separator but have a negligible effect on the weight and thickness of the separator, which may enhance the power and energy density of Li−S batteries. Here, we demonstrate that a CVD-G film with an area of 5 × 60 cm2 transferred onto a polypropylene (PP) separator can serve as an interlayer between the separator and the cathode. The CVD-G interlayer has a negligible increase in weight (∼0.15 μg cm−2) and thickness (∼0.6 nm) compared to a PP separator and is applicable with cathodes made of different sulfur/carbon (S/C) composites and electrolytes. The prepared Li−S battery with a two-layer CVD-G-coated PP separator delivers a specific capacity of 1460 mA h g−1 at 0.1 C (which is 111% of the specific capacity obtained with a bare PP separator), has a capacity decay rate of 0.026% per cycle after 1500 cycles at 0.5 C, and retains an open-circuit voltage (OCV) of 2.35 V for >500 h without decay. The intact and conductive CVD-G film on the PP separator minimizes the diffusion of soluble polysulfides and converts the soluble polysulfides to insoluble Li2S2/Li2S, leading to a suppression of the shuttle effect and an increase in sulfur utilization. Therefore, the Li−S battery prepared using the CVD-G-covered PP separator has an enhanced specific capacity, rate capability, and cycle performance and suppressed self-discharge compared to the analogous Li−S battery with a standard PP separator.

Figure 1. (a) Schematic of interlayers containing CVD-G film and photograph showing a PP separator covered with two layers of CVD-G with a total thickness of ∼0.6 nm and area of 5 × 60 cm2. (b, c) Scanning electron microscopy (SEM) images of a bare-PP and 2G-PP separator. (d) Transmission electron microscopy (TEM) image showing a cross section of 2G-PP. (e) Raman spectra of bare-PP and 2G-PP.

Experimental Section).37,38 Parts b and c of Figure 1 show the surfaces of a bare PP separator (bare-PP) and a PP separator covered with two layers of CVD-G film (2G-PP), respectively. The porous structure of PP is not visible after the addition of two layers of CVD-G. The cross-sectional transmission electron microscopy (TEM) image (Figure 1d) shows the interface between the two-layer CVD-G and the PP separator, and the Raman spectrum of 2G-PP (Figure 1e) shows intense and sharp peaks located at ∼1580 (G band) and ∼2700 (2D band) cm−1,40 indicating that the ∼0.6 nm thick two-layer CVD-G film closely adheres to the PP surface. It is important to note that one layer of CVD-G on the PP separator (1G-PP) can be broken (Figure S2) with an immeasurable amount of sheet resistance (Table S1), which is ascribable to the rough surface of bare-PP (rq = 15 nm, Figure S3). However, broken areas of CVD-G are rarely found with stacked layers of 2 and 5 (Figure S2). The sheet resistances of the two-layer and five-layer CVDG on PP are 360 and 180 ohm sq−1, respectively. The improved integrity of the CVD-G film with increased stacked-layer number is ascribed to the increased mechanical strength of the multilayer graphene.41 We further studied the morphology and sheet resistance of the CVD-G-covered PP separator and found that a CVD-G film with a stacked layer number of more than one was integrative and conductive after discharge−charge cycles in a Li−S cell (see discussion below).

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RESULTS AND DISCUSSION Interlayers formed by discrete nano-/microstructures, for instance, reduced graphene oxide (rGO),17 MoS2 flakes,39 and metal−organic framework,23 can limit the shuttle effect, yet at the expense of reduced volumetric and gravimetric energy because of the increased thickness and weight to the separator. CVD-G formed on copper (Cu) foil surface is the thinnest (∼0.33 nm) known conductive film and has a lateral size of tens of centimeters (Figure S1). The addition of a CVD-G layer has a negligible effect on the thickness and weight of the separator. For proof of concept, a CVD-G with an area of 5 × 60 cm2 was grown on Cu and transferred onto a PP separator (Figure 1a). Moreover, stacking single-layer CVD-G obtains graphene film with the number of layers in the range of 1−5 (Figure S2 and B

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) Galvanostatic discharge−charge profiles and (b) cycle performance of Li−S cells assembled with either bare-PP or 2G-PP measured with the addition of 2 wt % LiNO3 to the electrolyte. (c) Capacity decay rates measured in this work and reported in the literature that are plotted with respect to the areal mass weights of the interlayers coated on the respective separator. (d) Rate capability of Li−S cells assembled with either bare-PP or 2G-PP. (e) OCV profiles of the Li−S cells assembled with either bare-PP or 2G-PP. The S loading in the cathode was 1 mg cm−2.

Figure 3. Capacity decay rates and initial discharge capacities for six groups of Li−S cells to study whether the effects from the inclusion of 2G-PP on the electrochemical performance is dependent on the cathode materials or electrolyte additives (LiNO3).

demonstrating a more complete and kinetically favorable conversion of soluble Li2S4 to insoluble Li2S2/Li2S when 2GPP was applied. After 1500 discharge−charge cycles, the Li−S cell assembled with 2G-PP presented a capacity decay rate of 0.026% per cycle (Figure 2b). This is a considerable improvement over the cell containing bare-PP that gives a capacity decay rate of 0.050% per cycle. Furthermore, the capacity decay rate was lower than the values reported in previous work, which applied interlayer-coated PP separators (Figure 2c).17,20−35 The addition of two layers of CVD-G increased the thickness and weight of the PP separator by ∼0.6 nm and ∼0.15 μg cm−2 (Figure 2c and Table S2), respectively, which were negligible differences compared to the thickness

Figure 2a shows the discharge−charge profiles of the Li−S batteries assembled with bare-PP and 2G-PP, measured at 0.1 and 0.5 C (1 C = 1675 mA g−1). The CVD-G layers faced the cathode side when assembling the cell. The Li−S battery assembled with 2G-PP delivered initial specific discharge capacities of 1460 and 1035 mA h g−1 (with respect to the mass of S, both here and below) at 0.1 and 0.5 C, respectively. Both were ∼11% higher than the analogous battery assembled with bare-PP. The discharge plateaus at ∼2.1 V of the cells assembled with 2G-PP were longer than the plateaus of the cells with bare-PP (Figure 2a), and the hysteresis of the cell assembled with 2G-PP measured at 0.5 C was ∼0.3 V lower than that from the cell with bare-PP (Figure 2a, solid lines), C

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a, b) H-type glass cells assembled with either 2G-PP (a) or bare-PP (b). (c) UV−vis spectra of the R-tubes after keeping the cells still for 20 h. (d) Normalized specific capacities of Li−S containing two layers of PP separators, a two-layer CVD-G film placed between the S cathode and the PP separator (S-2G-PPs), between two PP separators (PP-2G-PP), and between the PP separator and Li anode (PPs-2G-Li). A high electrolyte/ sulfur ratio of 60 μL(electrolyte)/mg(sulfur) was used for the test of Figure 4d.

(∼25 μm) and weight (∼1.34 mg cm−2) of the separator or electrodes in the Li−S battery. In addition, the Li−S cell assembled with 2G-PP delievered higher specific capacities than that with bare-PP at different C-rates (Figure 2d). Also, the OCV of the Li−S cell assembled with 2G-PP stayed at 2.35 V for >500 h without decay, which is significantly longer than that with bare-PP (Figure 2e), indicating a limited self-discharge of the Li−S cell containing 2G-PP. These results indicate that the resulting cells with 2G-PP have enhanced specific capacity (Figure 2a), cycle performance (Figure 2b), and rate capability (Figure 2d) and suppressed self-discharge (Figure 2e) compared to the bare-PP analogues. To study whether the effects from the inclusion of 2G-PP on the electrochemical performance is dependent on the cathode materials or electrolyte additives (LiNO3), we assembled six groups of Li−S cells (Figure 3). In each group, the two cells contained the same type of cathode, anode, and electrolyte but different separators. In Figure 3, the black bar of each group represents the electrochemical performance of the cell assembled with bare-PP, and the red bar represents that of the cell assembled with 2G-PP. The Li−S cells with a series of cathode materials, including pure S (group 1), S/aMEGO (group 2; aMEGO is activated microwave exfoliated graphene oxide), and S/rGO (group 3), were assembled (Figure S4). Electrochemical tests of the prepared cells showed that, while the improvement rates varied, the specific capacities increased and the capacity decay rates decreased when the Li−S cells were assembled with 2G-PP compared to the cells assembled with bare-PP (Figure 3). The cells in group 4 were assembled with the same cathode of group 3, except for the exclusion of LiNO3, which is an important additive to the electrolyte.42 The cell assembled with 2G-PP in group 4 (without LiNO3) outputted an initial specific capacity of 1450 mA h g−1 and a capacity decay rate of 0.26% after 100 cycles at a discharge−charge rate of 0.1 C (Figure 3 and Figure S4c). For comparison, the cell assembled with barePP in group 4 had an initial specific capacity of 1310 mA h g−1 and a capacity decay rate of 0.41%. The improved cycle performance for the cell having 2G-PP is in accordance with the higher Coulombic efficiency (Figure S4c) that is attributed to the inhibited shuttle effect. The cells in group 5 and 6 were assembled with a S loading of ∼3.2 mg cm−2. The capacity decay rates measured on the cells with 2G-PP were lower than those with bare-PP and were comparable to or lower than those in the recently published works in which high S loadings were highlighted.39,43,44 These results indicate that the effect of 2GPP on the electrochemical performance of the Li−S cell is

independent of the types of cathode materials, S loadings, or electrolyte additives. To understand of the extent to which the CVD-G-coated PP separator has an effect on the electrochemical performance of the prepared Li−S batteries, two H-type cells (H-cell) were assembled with one H-cell being separated by 2G-PP (Figure 4a) and one being separated by bare-PP (Figure 4b). The tubes on the left-hand side (L-tubes) contained 0.5 M Li2S4 dissolved in 1,3-dioxolane (DOL)/dimethoxy ethane (DME) (v/v = 1:1), while the tubes on the right-hand side (R-tubes) contained only DOL/DME solvent. The cells were sealed and allowed to stand at room temperature for 20 h while intermittent photographs were taken. The solution in the Rtube of the H-cell assembled with 2G-PP remained clear after 20 h (Figure 4a). However, the color of the L-tube that was separated by bare-PP gradually turned yellow (Figure 4b), indicative of the diffusion of soluble Li2S4 through bare-PP. The UV−vis spectra of the solutions in the R-tubes after 20 h are shown in Figure 4c. Note that the solution in the R-tube that was separated by bare-PP was diluted by 1/6 before UV−vis spectroscopy measurement. Therefore, the concentration of Li2S4 in the R-tube separated by bare-PP was ∼18 times higher than that in the R-tube separated by 2G-PP, indicating that 2GPP can greatly reduce the diffusion of soluble polysulfides in the electrolyte. We also assembled Li−S cells that contained two layers of PP separators and a two-layer CVD-G film placed (a) between the S cathode and the PP separator (S-2G-PPs), (b) between two PP separators (PP-2G-PP), and (c) between the PP separator and Li anode (PPs-2G-Li). Figure 4d shows that the capacity decay rates of S-2G-PPs and PP-2G-PP are similar and are lower than the capacity decay rate of PPs-2G-Li. In the case of PPs-2G-Li, the polysulfides that were generated on the cathode side can diffuse through the separator and touch the conductive CVD-G surface that was electrically contacted with the Li anode. The electrochemical reduction of soluble polysulfides occurred on the anode side, which resulted in a shuttle effect. When the CVD-G film was placed between the separator and the S cathode, there was no electrical contact between the polysulfide and the Li anode. To study the effect of CVD-G interlayers on the Li+ transferance across the separator, we measured the Li+ diffusion coefficients for bare-PP, 1G-PP, 2G-PP, and 5G-PP by a series of cyclic voltagram (CV) measurements (Figure S5). The Randles−Sevcik equation was adopted,39,44 and then Li+ diffusion coefficients were calculated and are summarized in Table 1. The Li+ diffusion coefficients measured at peaks B (2.2−2.3 V) and C (1.9−2.1 V) involve the generation of D

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Summary of Li-Ion Diffusion Coefficients (DLi+) for bare-PP, 1G-PP, 2G-PP, and 5G-PP +

8

2

−1

DLi (×10 cm S )

at peak A at peak B at peak C

bare-PP

1G-PP

2G-PP

5G-PP

4.4(0) 0.5(5) 0.4(4)

4.1(0) 0.6(6) 0.9(0)

6.7(0) 0.7(3) 1.0(0)

4.5(0) 0.7(1) 0.6(2)

hystereses between the anodic peak (∼2.45 V) and the second cathodic peak (∼2.10 V) are 0.48, 0.4, 0.35, and 0.37 V, for cells assembled with bare-PP, 1G-PP, 2G-PP, and 5G-PP, respectively. The narrower faradaic peaks, smaller potential hysteresis, and charge-transfer resistance (Figure S8) indicate enhanced reaction kinetics of the S/Li2S conversion. The initial potentials of the second discharge plateau are 2.07, 2.04, 2.02, and 2.00 V for Li−S cells assembled with 2G-PP, 5G-PP, 1GPP, and bare-PP, respectively (Figure 5b). A higher initial potential for the Li−S cell having 2G-PP suggests a more favorable conversion of the soluble long-chain polysulfides to nonsoluble Li2S2,46 which, ultimately, leads to the highest cycle stability as compared to the cells assembled with bare-PP, 1GPP, or 5G-PP (Figure S9). The CVs of the Li−S cell assembled with 2G-PP and 5G-PP have two satellite peaks located at 2.54 and 1.90 V right after the anodic (∼2.45 V) and cathodic (∼2.10 V) peaks, respectively (Figure 5a). These satellites correspond to the two plateaus at the end of the charge and discharge curves (Figure S10). The two satellite peaks are also reversible (Figure S7) and are barely detectable in the CVs of the cells with barePP and 1G-PP. The satellite peaks indicate additional reactions that occur at the end of the charge and discharge processes. In other polysulfide-based flow batteries, similar extra plateaus were observed at the end of the charge and discharge curves47,48 and were ascribed to the conversion between soluble polysulfides in the electrolyte solution and insoluble Li2S2 and Li2S at the cathode surface, which acquires a larger overpotential. Because the CVD-G-stacked film interlayers on the separators are conductive (Table S1) and can be considered as flat electrodes connected in series with the cathode, the satellite peaks shown in the CV curves (Figure 5a) are attributed to the reversible conversion between soluble polysulfides in the electrolyte and insoluble Li2S2/Li2S on the CVD-G film. The long-chain polysulfides (Li2Sx, x = 4, 6, and 8) that are generated in the cathode during discharging are partially retained in the electrolyte at the end of the second

soluble polysulfides and insoluble polysulfides, respectively, at the cathode/separator interface. The building of polysulfide layers at the CVD-G-coated separator results in a lower Li+ diffusion coefficient than the value measured at peak A (2.4− 2.5 V). Nevertheless, the Li+ diffusion coefficients for the four types of separators are of the same order of magnitude, and the 2G-PP yields the highest value versus the other three types of separators. This result is in accordance with the conclusion of Lee and co-workers that a layer thickness to effectively prohibit Li+ diffusion using CVD-G layers is predicted to be more than six layers, independent of defect population.45 They also suggested that the point defects, which exist in the basal plane of the as-prepared CVD-graphene, are the place for Li+ to diffuse through. The point defects of CVD-G layers could be responsible for the Li+ transferance across the separator because the voids of CVD-G with diameter of larger than tens of nanometers were not found in SEM (Figure 1c and Figure S2) and the sheet resistance of cycled 2G-PP was comparable to the two-layer CVD-G transferred on polyethylene terephthalate (PET) substrate (Figure S6). Unfortunately, the concentration of point defect of CVD-G on PP separator is immeasurable because of the strong backgound of PP in Raman that overlaps the D band of graphene (Figure 1e). The CV curves of the Li−S cells show reversible cathodic and anodic peaks that correspond to the reversible conversion between elemental S and Li2S during discharge and charge (Figure 5a and Figure S7). The widths of the faradaic peaks from the cell containing bare-PP are ∼2-fold of the widths from the cells with CVD-G-covered PP separators. The potential

Figure 5. (a) CV curves of Li−S cells assembled with either bare-PP, 1G-PP, 2G-PP, or 5G-PP. (b) Discharge profiles of the initial cycle of the Li−S cells assembled with either bare-PP or separators covered with varying numbers of CVD-G layers. The diagram is rescaled to present the beginning of the second discharge plateau. (c) S/C atomic ratios and (d) SEM images acquired at the discharged state of cells on the surface of bare-PP, 1GPP, 2G-PP, and 5G-PP after 50 discharge−charge cycles. The insets of panel c show photographs of the separators after cycling. E

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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discharge plateau. These polysulfide species diffuse out of the cathode toward the anode side and shuttle between the cathode and the anode if a bare-PP is present. However, when a CVDG-covered PP separator is present, the soluble polysulfides are blocked and converted to Li2S2/Li2S at the CVD-G surface. Photographs of the separators after cycling (insets of Figure 5c) show that bare-PP has a white surface and the CVD-Gcovered PP separators have yellow surfaces. X-ray photoelectron spectra (Figure 5c and Figure S11) show that the S/C ratios of 1G-PP, 2G-PP, and 5G-PP are very close and are ∼2fold that of bare-PP. The SEM images (Figure 5d) show that the surface morphology of bare-PP is largely unchanged, that the one-layer CVD-G film is fragmented, and that the two-layer and five-layer CVD-G films remain integrated after 50 cycles. This result is consistent with the two-layer and five-layer CVDG having more complete transfer and lower sheet resistances than the one-layer CVD-G film on PP separator (Figure S2 and Table S1). In addition, a small resistance difference of a 2G-PP before and after cycling indicates a good mechanical strength and excellent adhesion of CVD-G layers on PP separator (Figure S6). After careful analysis of the CV and the morphologies of the cycled separators, we propose that the improved electrochemical performance of the Li−S batteries arises from the intact and conductive CVD-G film on the PP separator that reduces the diffusion of soluble polysulfides in the electrolyte solution between the cathode and the anode and converts the soluble polysulfides to insoluble Li2S2/Li2S to enhance sulfur utilization. The CVD-G film must have more than one stacked layer in order to be mechanically strong enough to completely transfer to the PP separator, which is critical for improved specific capacity and stability. Five layers of CVD-G film are more conductive than two layers on the PP separator (Table S1), which leads to the smallest charge-transfer impedance in the cathode (Figure S8). However, the addition of three more layers of CVD-G film to 2G-PP is costly, leads to higher potential hysteresis and Li2S2 nucleation overpotential (Figure 5a,b), and diminishes cycle stability of the Li−S cells (Figure S9). The Li+ diffusion coefficient for 2G-PP is of the same order of magnitude of bare-PP, and point defects of CVD-G could be responsible for the Li+ transference across the separator, yet the detailed analysis requires systematic studies in the future.

Research Article

EXPERIMENTAL SECTION

CVD-G Transfer. The CVD-G film on Cu foil was purchased from Wuxi Graphene Film Co., Ltd. Thermal release tape (TRT) (3M) was coated on one side of the graphene-covered Cu at a pressure of 0.2 MPa through a laminator at room temperature followed by etching the Cu foil with 0.5 M ammonium persulfate aqueous solution and subsequent rinsing in deionized water. A multilayer graphene film on TRT can be obtained by placing the graphene-covered Cu on the graphene-covered TRT through a laminator and subsequent etching of Cu and rinsing. The dried graphene on TRT was covered on a bare-PP separator, which was exposed to oxygen plasma (100 W) for 30 s through the laminator at a pressure of 0.2 MPa and temperature of 90 °C. The TRT was peeled off at room temperature, leaving the graphene film on PP separator. Sheet Resistance Measurement. The sheet resistance of CVD-G film on PP separator was measured by the van der Pauw method. Briefly, the graphene-covered PP samples were cut into squares with a lateral size of 1 cm followed by attaching Cu wires with a diameter of 200 μm at each corner with silver epoxy. The Rvertical and Rhorizontal were acquired by Keithley 4200 SCS, and the sheet resistance, Rs, was calculated by Rs = π(Rvertical + Rhorizontal)/(2 ln 2). Structure Characterization. SEM was performed on a JSM2100F (JEOL, Ltd.) operated at 5.0 kV. TEM images were obtained using a JEM-ARM200F (JEOL, Ltd.) at an accelerating voltage of 200 kV. A dual-beam FIB/S EM instrument (Helios 650, FEI) was used to prepare TEM samples for imaging the cross section of the 2G-PP. Raman spectra were obtained using an inVia Raman Microscope (Renishaw) with 532 nm incident laser excitation. X-ray photoelectron spectroscopy (XPS) analysis was conducted with a Thermo ESCALAB 250 instrument using a magnesium anode (monochromatic Kα X-rays at 1486.6 eV) as the source. UV−vis spectra were acquired by Maya2000Pro (Ocean Optics). S Cathodes Preparation. Three types of S electrodes (pure S, S/ aMEGO, and S/rGO) were used to investigate the performance of the graphene-covered separators, where aMEGO and rGO are activated microwave exfoliated graphene oxide and reduced graphene oxide, respectively. The pure S cathode contains 70 wt % sublimed S powder, 20 wt % Ketjen Black, and 10 wt % poly(vinylidene fluoride) (PVDF) binder coated on an aluminum current collector. The S/aMEGO and S/rGO composites contain 70 wt % S, and the cathodes contain 75 wt % S/C composites, 15 wt % Ketjen Black, and 10 wt % PVDF coated on an aluminum current collector. The S/rGO composite with 80 wt % S content was also prepared for a high S loading of ∼3.2 mg cm−2 to evaluate the effectiveness of the atom-thick interlayer on practical applications. Electrochemical Performance Tests. The electrochemical performance was examined using CR2032 coin-type cells assembled in Ar-filled glovebox. The electrolyte was 1.0 M LiTFSI in a mixture of DOL and DME (volume ratio 1:1) with 2 wt % LiNO3 or without LiNO3 as the additive. The average areal S loading was ∼1 mg cm−2 for the common electrochemical measurements. Cathodes with a high S loading of ∼3.2 mg cm−2 were also prepared to evaluate the potential of the atom-thick interlayer on the separator for practical applications. The electrolyte/sulfur ratios were 30 and 25 μL(electrolyte)/mg(sulfur) for Li−S cells with ∼1 and ∼3.2 mg(sulfur) cm−2, respectively. Galvanostatic discharge−charge behavior was performed using a CT2001A battery test system (LAND Electronic Co.). Cyclic voltagram and electrochemical impedance spectroscopy (EIS) measurements were performed on PARSTAT4000 electrochemical workstation. The CV data was collected at a scan rate of 0.05 mV s−1 between 1.5 and 2.8 V versus Li/Li+. EIS measurements were performed with ac amplitude of 5 mV in the frequency range of 0.01 Hz to 100 kHz. The CVD-Gcovered separators used for XPS analysis were assembled in Li−S cells with electrolyte containing 1.0 M LiClO4 instead of LiTFSI to avoid the S contributions from LiFTSI. Permeation Tests. The visible permeation tests were carried out in H-type glass cells to examine the capability of the CVD-G film in restraining the polysulfides. The left-hand tube contains DOL/DME solution without polysulfides, while the right-hand tube contains 0.5 M

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CONCLUSION In summary, we have shown that CVD-G film with an area of 5 × 60 cm2 can serve as an interlayer that reduces the diffusion of soluble polysulfides in the electrolyte between the cathode and the anode and enhances the utilization of sulfur in the cell. The optimized number of CVD-G layers was found to be two, which is the thinnest (∼0.6 nm) and lightest (∼0.15 μg cm−2) interlayer to date. The inclusion of a CVD-G film interlayer on the PP separator increases sulfur utilization and renders a low specific capacity decay rate and self-discharge. The CVD-Gcovered PP separator is compatible with cathodes consisting of different S/C composites and electrolytes either with or without LiNO 3 additive. There are few examples of modifications to Li−S batteries that enhance cycle performance and suppress self-discharge without sacrificing energy density. Moreover, in view of the versatility of the subnanometer-thick film for Li−S batteries, our strategy may be extended to design other nanomembranes for use in batteries and supercapacitors, making our work a valuable contribution to the field of electrochemical energy storage in general. F

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Li2S4 dissolved in DOL/DME. The Li2S4 solution was prepared by dissolving stoichiometric amounts of Li2S and S in DOL/DME at 60 °C for 12 h. DLi+ Measurement. Li-ion diffusion coefficients for bare-PP, 1GPP, 2G-PP, and 5G-PP were evaluated by a series of CVs with different scan rates and quantitatively calculated using Randles−Sevcik equation as given below,

Polyrhodanine@Cellulose for Advanced Lithium−Sulfur Batteries. Adv. Mater. 2015, 27, 6021−6028. (8) Li, G.; Sun, J.; Hou, W.; Jiang, S.; Huang, Y.; Geng, J. ThreeDimensional Porous Carbon Composites Containing High Sulfur Nanoparticle Content for High-Performance Lithium-Sulfur Batteries. Nat. Commun. 2016, 7, 10601. (9) Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A Highly Efficient Polysulfide Mediator for Lithium−Sulfur Batteries. Nat. Commun. 2015, 6, 5682. (10) Zhang, J.; Shi, Y.; Ding, Y.; Zhang, W.; Yu, G. In Situ Reactive Synthesis of Polypyrrole-MnO2 Coaxial Nanotubes as Sulfur Hosts for High-Performance Lithium−Sulfur Battery. Nano Lett. 2016, 16, 7276−7281. (11) Zhang, J.; Shi, Y.; Ding, Y.; Peng, L.; Zhang, W.; Yu, G. A Conductive Molecular Framework Derived Li2S/N,P-Codoped Carbon Cathode for Advanced Lithium−Sulfur Batteries. Adv. Energy Mater. 2017, 7, 1602876. (12) Cuisinier, M.; Cabelguen, P. E.; Adams, B. D.; Garsuch, A.; Balasubramanian, M.; Nazar, L. F. Unique Behaviour of Nonsolvents for Polysulphides in Lithium-Sulphur Batteries. Energy Environ. Sci. 2014, 7, 2697−2705. (13) Suo, L.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (14) Wu, F.; Lee, J. T.; Nitta, N.; Kim, H.; Borodin, O.; Yushin, G. Lithium Iodide as a Promising Electrolyte Additive for Lithium−Sulfur Batteries: Mechanisms of Performance Enhancement. Adv. Mater. 2015, 27, 101−108. (15) Su, Y.-S.; Fu, Y.; Cochell, T.; Manthiram, A. A Strategic Approach to Recharging Lithium-Sulphur Batteries for Long Cycle Life. Nat. Commun. 2013, 4, 2985. (16) Zhou, G.; Pei, S.; Li, L.; Wang, D.-W.; Wang, S.; Huang, K.; Yin, L.-C.; Li, F.; Cheng, H.-M. A Graphene-Pure-Sulfur Sandwich Structure for Ultrafast, Long-Life Lithium−Sulfur Batteries. Adv. Mater. 2014, 26, 625−631. (17) Huang, J. Q.; Zhuang, T. Z.; Zhang, Q.; Peng, H. J.; Chen, C. M.; Wei, F. Permselective Graphene Oxide Membrane for Highly Stable and Anti-Self-Discharge Lithium-Sulfur Batteries. ACS Nano 2015, 9, 3002−3011. (18) Su, Y.-S.; Manthiram, A. Lithium−Sulphur Batteries with a Microporous Carbon Paper as a Bifunctional Interlayer. Nat. Commun. 2012, 3, 1166. (19) Huang, C.; Xiao, J.; Shao, Y.; Zheng, J.; Bennett, W. D.; Lu, D.; Saraf, L. V.; Engelhard, M.; Ji, L.; Zhang, J.; Li, X.; Graff, G. L.; Liu, J. Manipulating Surface Reactions in Lithium−Sulphur Batteries Using Hybrid Anode Structures. Nat. Commun. 2014, 5, 3015. (20) Zhou, G.; Li, L.; Wang, D.-W.; Shan, X.-y.; Pei, S.; Li, F.; Cheng, H.-M. A Flexible Sulfur-Graphene-Polypropylene Separator Integrated Electrode for Advanced Li−S Batteries. Adv. Mater. 2015, 27, 641− 647. (21) Chung, S.-H.; Manthiram, A. High-Performance Li−S Batteries with an Ultra-Lightweight MWCNT-Coated Separator. J. Phys. Chem. Lett. 2014, 5, 1978−1983. (22) Wang, G.; Lai, Y.; Zhang, Z.; Li, J.; Zhang, Z. Enhanced Rate Capability and Cycle Stability of Lithium-Sulfur Batteries with a Bifunctional MCNT@PEG-Modified Separator. J. Mater. Chem. A 2015, 3, 7139−7144. (23) Bai, S.; Liu, X.; Zhu, K.; Wu, S.; Zhou, H. Metal−Organic Framework-Based Separator for Lithium−Sulfur Batteries. Nat. Energy 2016, 1, 16094. (24) Gu, X.; Lai, C.; Liu, F.; Yang, W.; Hou, Y.; Zhang, S. A Conductive Interwoven Bamboo Carbon Fiber Membrane for Li-S Batteries. J. Mater. Chem. A 2015, 3, 9502−9509. (25) Yao, H.; Yan, K.; Li, W.; Zheng, G.; Kong, D.; Seh, Z. W.; Narasimhan, V. K.; Liang, Z.; Cui, Y. Improved Lithium-Sulfur Batteries with a Conductive Coating on the Separator to Prevent the Accumulation of Inactive S-Related Species at the Cathode-Separator Interface. Energy Environ. Sci. 2014, 7, 3381−3390.

Ip = (2.69 × 105)n1.5ADLi +0.5C Li +v 0.5 in which Ip (A) stands for the peak current in amperes, n is the number of electrons in the reaction (n = 2 for Li−S battery), A (cm2) is the electrode area, CLi+ (mol mL−1) is the Li-ion concentration, and v (V s−1) represents the scanning rate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14195. Photographs, SEM and AFM images, and sheet resistance of CVD−graphene films; XPS data; CV data; EIS data; and galvanostatic discharge−charge data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yu-Guo Guo: 0000-0003-0322-8476 Hengxing Ji: 0000-0003-2851-9878 Li-Jun Wan: 0000-0002-0656-0936 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate funding support from the Natural Science Foundation of China (21373197 and 11474265), support from the 100 Talents Program of the Chinese Academy of Sciences, USTC Startup, the Fundamental Research Funds for the Central Universities (WK3430000003), and iChEM. X.K. thanks the Anhui Provincial Natural Science Foundation for support (Grant 1508085QE103).

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REFERENCES

(1) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (2) Sun, Y.; Liu, N.; Cui, Y. Promises and Challenges of Nanomaterials for Lithium-Based Rechargeable Batteries. Nat. Energy 2016, 1, 16071. (3) Manthiram, A.; Chung, S. H.; Zu, C. X. Lithium-Sulfur Batteries: Progress and Prospects. Adv. Mater. 2015, 27, 1980−2006. (4) Zhang, J.; Huang, H.; Bae, J.; Chung, S.; Zhang, W.; Manthiram, A.; Yu, G. Nanostructured Host Materials for Trapping Sulfur in Rechargeable Li-S Batteries. Small Methods 2017, 1, 1700279. (5) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500−506. (6) Zhou, Y.; Zhou, C.; Li, Q.; Yan, C.; Han, B.; Xia, K.; Gao, Q.; Wu, J. Enabling Prominent High-Rate and Cycle Performances in One Lithium-Sulfur Battery: Designing Permselective Gateways for Li+ Transportation in Holey-CNT/S Cathodes. Adv. Mater. 2015, 27, 3774−3781. (7) Pang, Q.; Tang, J.; Huang, H.; Liang, X.; Hart, C.; Tam, K. C.; Nazar, L. F. A Nitrogen and Sulfur Dual-Doped Carbon Derived from G

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Interface for Highly Efficient Lithium−Sulfur Batteries. Adv. Mater. 2016, 28, 9551−9558. (45) Yao, F.; Gunes, F.; Ta, H. Q.; Lee, S. M.; Chae, S. J.; Sheem, K. Y.; Cojocaru, C. S.; Xie, S. S.; Lee, Y. H. Diffusion Mechanism of Lithium Ion through Basal Plane of Layered Graphene. J. Am. Chem. Soc. 2012, 134, 8646−8654. (46) Ji, X.; Nazar, L. F. Advances in Li−S Batteries. J. Mater. Chem. 2010, 20, 9821−9826. (47) Chen, H.; Zou, Q.; Liang, Z.; Liu, H.; Li, Q.; Lu, Y.-C. SulphurImpregnated Flow Cathode to Enable High-Energy-Density Lithium Flow Batteries. Nat. Commun. 2015, 6, 5877. (48) Fan, F. Y.; Woodford, W. H.; Li, Z.; Baram, N.; Smith, K. C.; Helal, A.; McKinley, G. H.; Carter, W. C.; Chiang, Y.-M. Polysulfide Flow Batteries Enabled by Percolating Nanoscale Conductor Networks. Nano Lett. 2014, 14, 2210−2218.

(26) Chung, S.-H.; Manthiram, A. Bifunctional Separator with a Light-Weight Carbon-Coating for Dynamically and Statically Stable Lithium-Sulfur Batteries. Adv. Funct. Mater. 2014, 24, 5299−5306. (27) Balach, J.; Jaumann, T.; Klose, M.; Oswald, S.; Eckert, J.; Giebeler, L. Functional Mesoporous Carbon-Coated Separator for Long-Life, High-Energy Lithium-Sulfur Batteries. Adv. Funct. Mater. 2015, 25, 5285−5291. (28) Chung, S.-H.; Han, P.; Singhal, R.; Kalra, V.; Manthiram, A. Electrochemically Stable Rechargeable Lithium-Sulfur Batteries with a Microporous Carbon Nanofiber Filter for Polysulfide. Adv. Energy Mater. 2015, 5, 1500738−1500749. (29) Balach, J.; Jaumann, T.; Klose, M.; Oswald, S.; Eckert, J.; Giebeler, L. Mesoporous Carbon Interlayers with Tailored Pore Volume as Polysulfide Reservoir for High-Energy Lithium-Sulfur Batteries. J. Phys. Chem. C 2015, 119, 4580−4587. (30) Xiao, Z.; Yang, Z.; Wang, L.; Nie, H.; Zhong, M. e.; Lai, Q.; Xu, X.; Zhang, L.; Huang, S. A Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life Lithium−Sulfur Batteries. Adv. Mater. 2015, 27, 2891−2900. (31) Wei, H.; Ma, J.; Li, B.; Zuo, Y.; Xia, D. Enhanced Cycle Performance of Lithium−Sulfur Batteries Using a Separator Modified with a PVDF-C Layer. ACS Appl. Mater. Interfaces 2014, 6, 20276− 20281. (32) Chung, S.-H.; Manthiram, A. A Polyethylene Glycol-Supported Microporous Carbon Coating as a Polysulfide Trap for Utilizing Pure Sulfur Cathodes in Lithium−Sulfur Batteries. Adv. Mater. 2014, 26, 7352−7357. (33) Kim, J. H.; Seo, J.; Choi, J.; Shin, D.; Carter, M.; Jeon, Y.; Wang, C.; Hu, L.; Paik, U. Synergistic Ultrathin Functional Polymer-Coated Carbon Nanotube Interlayer for High Performance Lithium−Sulfur Batteries. ACS Appl. Mater. Interfaces 2016, 8, 20092−20099. (34) Kim, M. S.; Ma, L.; Choudhury, S.; Archer, L. A. Multifunctional Separator Coatings for High-Performance Lithium−Sulfur Batteries. Adv. Mater. Interfaces 2016, 3, 1600450. (35) Kim, M. S.; Ma, L.; Choudhury, S.; Moganty, S. S.; Wei, S.; Archer, L. A. Fabricating Multifunctional Nanoparticle Membranes by a Fast Layer-by-Layer Langmuir-Blodgett Process: Application in Lithium-Sulfur Batteries. J. Mater. Chem. A 2016, 4, 14709−14719. (36) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (37) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (38) Ji, H.; Zhao, X.; Qiao, Z.; Jung, J.; Zhu, Y.; Lu, Y.; Zhang, L. L.; MacDonald, A. H.; Ruoff, R. S. Capacitance of Carbon-Based Electrical Double-Layer Capacitors. Nat. Commun. 2014, 5, 3317. (39) Ghazi, Z. A.; He, X.; Khattak, A. M.; Khan, N. A.; Liang, B.; Iqbal, A.; Wang, J.; Sin, H.; Li, L.; Tang, Z. MoS2/Celgard Separator as Efficient Polysulfide Barrier for Long-Life Lithium−Sulfur Batteries. Adv. Mater. 2017, 29, 1606817. (40) Gao, L.; Ni, G. X.; Liu, Y.; Liu, B.; Castro Neto, A. H.; Loh, K. P. Face-to-Face Transfer of Wafer-Scale Graphene Films. Nature 2014, 505, 190−194. (41) Zhang, Y.; Pan, C. Measurements of Mechanical Properties and Number of Layers of Graphene from Nano-Indentation. Diamond Relat. Mater. 2012, 24, 1−5. (42) Aurbach, D.; Pollak, E.; Elazari, R.; Salitra, G.; Kelley, C. S.; Affinito, J. On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li−Sulfur Batteries. J. Electrochem. Soc. 2009, 156, A694−A702. (43) Sun, J.; Sun, Y.; Pasta, M.; Zhou, G.; Li, Y.; Liu, W.; Xiong, F.; Cui, Y. Entrapment of Polysulfides by a Black-Phosphorus-Modified Separator for Lithium−Sulfur Batteries. Adv. Mater. 2016, 28, 9797− 9803. (44) Peng, H.-J.; Zhang, Z.-W.; Huang, J.-Q.; Zhang, G.; Xie, J.; Xu, W.-T.; Shi, J.-L.; Chen, X.; Cheng, X.-B.; Zhang, Q. A Cooperative H

DOI: 10.1021/acsami.7b14195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX